In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of a Device to Device (D2D) proximity service is discussed as a new function in Release 12 and later (see Non Patent Document 1).
In a mobile communication system, as a technology for reducing an overhead in a header included in a PDU (Protocol Data Unit), a header compression technology is discussed. Specifically, a non-compressed header includes a static header in which an invariable value is stored and a dynamic header in which a variable value is stored. On the other hand, a compressed header includes identification information (CID: Context ID) for identifying the static header and a dynamic header. That is, in the header compression technology, when the static header is replaced by the identification information, an overhead of the header is reduced.
In the above-described 3GPP, it is discussed to use the header compression technology also in the above-described D2D proximity service (D2D ProSe). However, in the D2D proximity service, a timing at which a receiving entity (PDCP layer) is formed in the receiving terminal differs from a timing at which a transmitting entity (PDCP layer) is formed in the transmitting terminal. Therefore, when the compressed header is received without receiving the non-compressed header before the receiving entity is formed, the receiving terminal is not capable of decode the compressed header.